1. Field of the Invention
The present invention relates to methods for manufacturing sensors using semiconductors, and more particularly, to methods for manufacturing sensors that employ semiconductors for pressure sensors, acceleration sensors, or the like.
2. Description of the Related Art
As shown in FIG. 1, a pressure sensor 31 has a diaphragm 33, which serves as a pressure receiving surface (detecting portion). The diaphragm 33 is employed to produce displacement or distortion that is proportional to pressure. The diaphragm 33 is formed using the crystal anisotropic etching method to partially etch a silicone substrate 32 at one side, which has a (100) plane. The diaphragm 33 has a surface on which a plurality of dispersion strain gauges 34 are arranged. The diaphragm 33 is bent by pressure. The dispersion strain gauges 34 are constructed so that their resistance values change in accordance with the bending of the diaphragm 33. The detection of changes in the resistance values of the dispersion strain gauges enables detection of the pressure.
To manufacture the pressure sensor 31 shown in FIG. 1, a silicon substrate 32 (wafer) having a thickness of 200 to 300 microns is normally used. It is preferable that the diaphragm 33 be wide and thin so as to enable the pressure sensor 31 to have a high detecting sensitivity. However, to form such a diaphragm 33, it is necessary that the substrate be provided with an opening having a relatively large dimension W1 for the following reasons. To carry out anisotropic etching on a thick silicone substrate 31, an etching hole having an opening angle corresponding to the (111) plane must be provided. The opening angle necessitates an opening having a relatively large dimension. This hinders the production of a compact pressure sensor 31.
To produce a dispersion strain gage 34, the resistance of which changes dramatically, the silicon substrate 32 having n-type conductivity and the dispersion strain gage 34 having p-type conductivity are used. In this case, the etching time affects control of the thickness of the diaphragm 33. Thus, it is difficult to form a diaphragm 33 having uniform thickness.
As shown in FIG. 2, another type of pressure sensor 41 has a diaphragm 43, which is formed on a silicon substrate 42. The diaphragm 43 is made of a silicon nitride film (Si.sub.3 N.sub.4) and has a square shape. The silicon substrate 42 has a pyramid-like well that is located right beneath the diaphragm 43 and formed by performing anisotropic etching. The well and the diaphragm 43 define a vacuum reference pressure chamber 44. On the opposite side of the reference pressure chamber 44, the diaphragm 43 has a surface on which a plurality of strain gages 43 are arranged. The strain gages 45 are formed by the deposition of polysilicon. The diaphragm 43 bends in accordance with the difference between the external pressure and the pressure of the reference pressure chamber 44. Since the reference pressure chamber 44 is under vacuum pressure, the pressure sensor 44 can detect absolute pressure.
The pressure sensor 41 is produced in accordance with the following manufacturing process.
Step 1 (refer to FIG. 3A): A first nitride film 51 made of Si.sub.3 N.sub.4 is applied to the surface of the silicon substrate 42. A portion of the first nitride film is removed to form a square window 51a.
Step 2 (refer to FIG. 3B): An etching channel 52 made of polycrystalline silicon is applied so as to encompass the window 51a.
Step 3 (refer to FIG. 3C): A second nitride film 53 made of Si.sub.3 N.sub.4 is applied to the entire surface of the etching channel 52 and the first nitride film 51. Strain gages 45 made of polycrystalline silicon are formed on part of the second nitride film 53 that correspond to the formation area of the diaphragm 43. To cover the strain gages 45, a third nitride film 54 made of Si.sub.3 N.sub.4 is applied to the entire surface of the second nitride film 53. At the peripheral region of the formation region of the diaphragm 43, etching bores 55 are formed extending through the second and third nitride films 52, 53.
Step 4 (refer to FIG. 3D): An anisotropic etching solution is poured into the bores 55 to perform undercut etching (anisotropic etching) of the silicon substrate 42. The etching channel 52 is gradually removed by the etching solution and a well is gradually formed in the surface of the silicon substrate 41. Etching is completed when the entire etching channel 52 is removed and the pyramid-like well is formed in the silicon substrate 42. The well has four sidewalls that has the (111) plane.
Step 5 (refer to FIG. 2): The plasma chemical vapor-deposited (CVD) method is used to seal the bores 55. This applies a fourth nitride film 56 made of Si.sub.3 N.sub.4 on the entire surface of the device. The pressure sensor 41 having a vacuum reference pressure chamber 44 is produced in this manner.
The pressure sensor 41 of FIG. 2 is more compact than the pressure sensor 31 of FIG. 1 since the reference pressure chambers 44 are pyramid-like. However, it is difficult to securely seal the bores 55 so as to maintain the reference pressure chambers 44 in a vacuum state.
As shown in FIG. 4, an acceleration sensor 61 has a detecting portion 63 defined in a silicon substrate. The silicon substrate 62 is formed in a rectangular-pillar shape to have a (100) plane. The detecting portion 63 includes a cantilever 65 and a mass (inertial mass) 64 serving as a weight, which are formed integrally at the middle region of the substrate by removing part of the upper and lower surfaces of the silicon substrate 62 with the anisotropic etching. The cantilever 65 extends horizontally from the inner end surface of the substrate 61 that is formed by etching. The cantilever 65 has a surface on which a plurality of dispersion strain gages 66 are arranged. The mass 64 is supported so that the distal end of the cantilever 65 is maintained in a hollow state. When acceleration causes displacement of the mass 64, the cantilever 65 is bent and causes a change in the resistance value of the dispersion strain gages 66. Detection of changes in the resistance value of the dispersion strain gages 66 enables detection of the acceleration.
To manufacture the acceleration sensor 61 of FIG. 4, a silicon substrate 62 (wafer) having a thickness of 200 to 300 microns is normally used. The cantilever 65 must be relatively long and thin to allow the acceleration sensor 61 to have high detecting sensitivity. However, to form such a cantilever 65, it is necessary that the substrate 62 be provided with a relatively large dimension W2. To carry out anisotropic etching on the thick silicone substrate 62, an etching hole having an opening angle corresponding to the (111) plane must be provided. The opening angle necessitates an opening having a relatively large dimension. This increases the size of the substrate 62 and hinders the production of a compact acceleration sensor 61.